The most important experimental technique of particle physics: accelerating electrically charged particles with the help of elektric forces, make them collide with each other and, from the result of the collision, draw conclusions about the properties of elementary particles and their interactions.

The branch of physics that deals with particles that are, to the best
of today's knowledge, not made up of more fundamental sub-units,
for instance with electrons,
quarks or
neutrinos. Also
included is the study of some species of particles that do have more
elementary constitutents, such as
protons or
neutrons, but not of
larger systems such as atomic nuclei (that would be nuclear physics) or, even worse, whole atoms. On the other hand, the question whether or not the particles
nowadays thought to be elementary really are elementary or are, for instance,
different manifestions of one and the same species of tiny
string does fall within
the purview of particle physics.

Synonym: Sum over histories. A technique for performing calculations in quantum theory. Roughly speaking, the probability for a certain outcome (for instance, a particle reaching location A at time t) is calculated by performing a sum over all possible ways in which this particular outcome can come about.

Basic principle of quantum theory stating that no two fermions
can be in exactly the same state - for instance: no two fermions with
identical properties can be at the same location. Formulated by the
physicist Wolfgang Pauli.

Electrons
are fermions, and the Pauli exclusion principle plays a crucial role in
bringing about the properties of matter as we know them: It is
responsible for the fact that the electrons of atoms do not all cluster together in the lowest-energy state close to the atomic nucleus,
but instead spread out, occupying different states. This is what gives
atoms their shell structure, responsible for different atoms' different
chemical properties.

For planetary orbits, there is a minute difference between the predictions of Newtonian gravity and general relativity. For instance, in Newton's theory, the orbital curve of a lonely planet orbiting a star is an ellipse. In general relativity, it is a kind of rosetta curve, corresponding to a partial ellipse that, in toto, shifts a bit with each additional orbit. The shift can be defined by looking at the point on each orbit closest to the sun, each perihelion, and the additional relativistic shift is, hence, called relativistic perihelion shift or relativistic perihelion advance. A picture can be seen on the page A planet goes astray in the chapter General relativity of Elementary Einstein.

Perimeter Institute for Theoretical Physics

Privately funded institute for basic research in theoretical physics, located in Waterloo, Canada. Currently, the main areas of research are quantum computing, the foundations of quantum theory, and quantum gravity.

When light shines onto a metal, it can knock electrons out of the metal's atoms. This is the photoelectric effect, and its properties - how does the number andenergy of the electrons depend on the frequency and intensity of the light? - can only be explained if one accepts that light is no mere electromagnetic wave, but somehow made up of some kind of
light particles. With this postulate, Einstein, in 1905, paved the way for the later development of quantum mechanics.

In a certain distance from a spherically symmetric black hole, the deflection of light because of the black hole's gravity is so great that light can move on closed circular orbits - photons (light particles) can, at this distance, orbit the black hole like a planet the sun. This particular distance is called the photon radius.

An observer at rest at this distance can see the back of his or her
own head (or at least a small region thereof), as the photons emitted
by the back of the head travel once around the black hole and fly
directly into his or her eyes.

Natural unit of mass that can be obtained by combining the
fundamental natural constants that govern space-time, the strength of
gravity and the quantum world: the gravitational constant, Planck's constant and the speed of light.
Compared with the masses we're used to in everday life, the Planck mass
is rather small, a mere 2 hundredth of a thousandth of a gram. However,
if this mass is concentrated in a single elementary particle then, in addition to the effects of quantum theory, the effects of general relativity should become important, in short: such a particle could only be described adequately using a theory of quantum gravity.

Fundamental constant of quantum theory; of the dimension energy times time. For instance, the energy of a single photon is equal to Planck's constant times the photon's frequency. Abbreviated as h in formulae.

Planck's radiation law

The fundamental law governing the properties of the simplest form of thermal radiation - that emitted by a blackbody. It describes the spectrum of such radiation in terms of universal constants and a single parameter - the body's temperature. The result is also called a blackbody spectrum.

plane

A surface within which the axioms of Euclidean geometry (synonym: plane geometry) hold - the rules of geometry as they are taught in high school, with well-known formulae such as Pythagoras' theorem and "the perimeter of a circle is 2 times pi times its radius" hold.

planet

Planets are not-too-small companions of a star that are not stars themselves (nor ever were stars). In our solar system, the planets are, listed from the one closest to the sun to the one farthest: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. As of August 2006, Pluto, which used to be a proper planet, is officially a "dwarf planet". In the night sky, the distinguishing characteristic of planets is that they move around relative to the unchanging background of stars - which gave them their name, loosely translated from Greek as "wanderers".

plasma

State of matter in which
a sizeable number of atoms have lost electrons to make for an electrically interacting mixture of electrons and rump atoms, or even
naked atomic nuclei flying around.

Elementary "building block" of geometrical entities such as surfaces or more general spaces. For instance, a surface is the set of all its points, of all possible locations on the surface, and all geometrical object in that surface are defined by the points that belong to them - for instance, a line on the surface is the set of (infinitely many) points.

polarization

Waves that are especially simple can be completely described by stating the direction in which they propagate, their speed of propagation, frequency, and amplitude.
But there are also simple wave where these quantities are not
sufficient for a complete description - for these waves, the
oscillation has an orientation in space. This orientation, which is
called polarization, needs to be specified as well.

For example, for electromagnetic waves, the polarization describes the directions of the
electric and the magnetic fields. For gravitational waves,
the polarization describes the orientation of the two orthogonal
directions in which distances are maximally stretched and squeezed as
the gravitational wave passes.

For situations in which gravity is very weak, general relativity and
Newton's theory of gravity
lead to very similar predictions for the motion of bodies and the
propagation of light. Such situations can be described by starting out
with the Newtonian description and then, step by step, adding
correction terms that take into account the effects of general
relativity. The post-Newtonian formalism is a method for performing
those step-by-step corrections. As the correction terms are ordered in
a systematic way (the largest effects are called "of first
post-Newtonian order, 1pN", the next smallest ones of second order, and
so on), the progression of ever smaller corrections is also called the
post-Newtonian expansion.

The post-Newtonian formalism is used to describe planetary motion as well as the propagation of light within our solar system; it is also used to describe the relativistic effects within binary neutron star systems.

potential energy

When an object is being acted upon by a force like the electric or gravitational force, then it can be assigned an energy that depends only upon its location relative to the source of the force. This energy is called the potential energy - "potential" as it can easily be transformed into kinetic energy, energy associated with the object's motion: As the object yields to the pull or push of the force, its potential energy decreases while the energy associated with its motion increases.

pressure

A measure for the strength of the resistance with which matter
(for instance a gas)
resists attempts to decrease the volume it occupies.